There is a need in the coatings industry for high strength, high modulus, chemically-resistant coatings. For high modulus, the coatings must exhibit exceptional hardness, while for high strength, they must have good resistance to impact. As to chemical resistance, it is particularly desirable for automotive coatings to have excellent acid-etch resistance. This invention is directed toward the above objective. Crosslinkable polyester coatings resins are synthesized and formulated into industrial baking enamels from which surface coatings can be obtained, the cured films exhibiting a unique combination of hardness (pencil hardness &gt;4H) and impact resistance (160 lb-in) as well as exceptionally high acid-etch resistance.
It has been demonstrated that thermotropic liquid crystal polymers (LCPs) possess many advantages over conventional thermoplastics. The self-reinforcement of the ordered polymeric molecules in the liquid crystalline (LC) state generally yield thermoplastics with exceptionally high strength and high modulus. In addition, LCPs have also been shown to have excellent solvent resistance, chemical resistance (e.g., acid, base, detergent), and weatherability. As described above, these properties are highly desirable in surface coatings. Recently, coating scientists have attempted to apply the LCP technology to the coatings art.
U.S. Pat. No. 5,043,192 (Jones) discloses the application of liquid crystalline polyesters to coatings which display high hardness and high impact resistance. In one of the examples, linear oligoester diols were prepared and modified with p-hydroxybenzoic acid (PHBA) to yield LC oligoesters. The PHBA diol mole ratio varied from 2.1/1 to 13/1. It was suggested that the excess of PHBA formed repeating p-oxybenzoyl LC segments in the oligoester chain ends. The resulting resins, however, were highly colored (i.e., brownish). The use of large quantities of the expensive raw material, PHBA, also made the resins commercially impractical.
European Patent Application No. 419088 discloses non liquid crystalline esterphenol-capped liquid polymer and polyol compositions in combination with an amino crosslinking agent which provided films having superior properties. The resins were prepared by following a procedure similar to that of Jones at a lower reaction temperature (i.e., &lt;200.degree. C.) A PHB aliphatic hydroxyl equivalent ratio of 1/1 was used mostly, although it was suggested the ratio could be ranged from about 0.05 to about 1.25. This reference teaches that the coatings showed improved hardness and impact resistance.
Curable resin compositions modified with hydroxybenzoic acid had also been reported by others. U.S. Pat. No. 2,993,873 disclosed that drying times and coating properties of oil-modified alkyd resins could be improved by replacing part of unsaturated fatty acids with hydroxybenzoic acid in the resin formulations. The coatings were cured by air dry or baking without the presence of a crosslinking agent. U.S. Pat. Nos. 4,267,239 and 4,298,658 describe the modification of alkyd resins with PHBA. The resulting resins could be rapidly cured at ambient temperatures with isocyanates in the presence of a tertiary amine vapor. U.S. Pat. Nos. 4,343,839 and 3,836,491 disclose a coating composition which is rapidly curable at room temperature in the presence of a tertiary amine catalyst vapor. The coating compositions comprised phenolic terminated polyesters and multi-isocyanate curing agents. U.S. Pat. No. 4,331,782 discloses the improved synthesis of a phenol-functional polyester polymer which utilizes a preformed adduct of a hydroxybenzoic acid and an epoxy compound. Japanese Patents No. 75 40,629, 76 56,839, 76 44,130, and 77 73,929 disclose powder coating compositions containing phenolic hydroxy end groups. These resins had high softening points and were applied to the surface as powders.
In an effort to raise the softening point of the melamine type crosslinking agent for powder coatings, U.S. Pat. No. 4,189,421 taught the synthesis of solid addition products having a softening point above 100.degree. F. by the reaction of a monohydroxy, single-ring aromatic compound and a hexakis(alkoxymethyl)amino-triazine ring (e.g., hexamethoxymethylmelamine, HMMM). They found the phenol compound not only could react with HMMM to form ether linkages (O-alkylation), but it could also form methylene bridges at the phenol ortho- or para- position (C-alkylation). The extent of both reactions was essentially equal. Further ring closure of the resulting product could also have occurred.
Polyester and acrylic resins are the two most common polymers used for coatings binders. They are synthesized by different types of polymerization mechanisms, and their chemical structures are basically different. While the ester linkages of polyesters lead to flexible coatings having good impact resistance, they are also prone to hydrolysis which results in poorer enamel stability and coating weatherability. On the other hand, the carbon-carbon bonds formed in acrylic backbones provide coatings with good weatherability, but they also attribute to the coating's brittleness, i.e., poor impact resistance.
These two types of resins may be used according to the desired properties of various coating applications. Nevertheless, it is often desired to have coatings having good impact resistance as well as weatherability, such as those used in automotive finishes. As a result, it is of interest to the coating industry to try to combine and take advantage of these two technologies. There are many examples of resin or solution blends of polyester and acrylic resins in coating formulations. The blends can be formulated with a crosslinker to form thermosetting coatings which lead to an interpenetrating network of polyester and acrylic when cured. In the present invention, phenol-functional (PF) polyesters are utilized to blend with hydroxyl-functional acrylics to form thermosetting coatings which exhibit significantly improved properties.
The phenol-functional polyester resins utilized in this invention are described in copending application Ser. Nos. 07/883,079; 07/883,509; 07/954,992; and 07/954,993, incorporated herein by reference. Coating compositions comprising blends of polyesters and acrylics are the subject of the following U.S. Pat. No.: 4,076,766; 4,322,508; 4,338,379; 4,397,989; 4,751,267; and 4,716,200.